1. Field of the Invention
This invention relates to a semiconductor element such as a field effect thin film transistor, and the like, and more particularly to a semiconductor element of which the main part is constituted of a polycrystalline silicon thin film semiconductor layer.
2. Description of the Prior Art
Recently, for constructing a scanning circuit portion of an image reading device for use in image reading, such as a one-dimensional photosensor made in a continuous length or a two-dimensional photosensor of an enlarged area, or for constructing a driving circuit of an image display device utilizing liquid crystal (abbreviated as LC); electrochromic material (abbreviated as EC) or electroluminescence material (abbreviated as EL), it has been proposed to form a field effect thin film transistor by using as the base material a silicon thin film formed on a certain substrate, corresponding in size to the increased area of such display portions.
Such a silicon thin film is desired to be polycrystalline rather than amorphous for realization of a large scale image reading device or image display device to improve speed and function. In spite of the desirability of increasing that the effective carrier mobility (.mu.eff) of a silicon thin film as a base material for formation of a scanning circuit portion of such a high speed, high function reading device or driving circuit portion, the amorphous silicon thin film obtained by the ordinary discharge decomposition method is at most 0.1 cm.sup.2 /V.sec., which is by far inferior to MOS type transistors of single crystalline silicon. The small carrier mobility (.mu.eff) due to the Hall mobility in amorphous silicon and a large change with lapse of time are inherent characteristics of an amorphous silicon thin film, and therefore, amorphous silicon thin films can not take advantage of the ease of preparation and low production costs offered by polycrystalline thin films.
In contrast, a polycrystalline silicon thin film has a Hall mobility by far greater than that of an amorphous silicon thin film as can be seen from measured data. Theoretically, it is quite probable that a polycrystalline silicon thin film may be prepared with an even larger value of mobility (.mu.eff) than that presently obtained.
In the prior art, various methods for preparing a polycrystalline silicon film over a large area on a given substrate are well known, such as Chemical Vapour Deposition (CVD) method, Low Pressure Chemical Vapour Deposition (LPCVD) method, Molecular Beam Epitaxy (MBE) method, Ion Plating (IP) method, Glow Discharge (GD) method, etc.
According to any one of these methods, it has been known that it is possible to prepare a polycrystalline silicon film on a substrate of a large area although the substrate temperature may differ depending on the methods.
However, under the present state of the art, a semiconductor element or a semiconductor device with its main part constituted of a semiconductor layer of a polycrystalline silicon thin film prepared by these methods cannot exhibit sufficiently desired characteristics and reliability.